1. Field of the Invention:
The present invention relates to a rotary powder compression molding machine used for producing tablets and the like.
2. Description of the Prior Art:
A rotary powder compression molding machine called a tablet machine has been used for producing medicine, food, bubble bath and the like in a tablet shape. The rotary powder compression molding machine comprises a compression molding machine body having a turntable provided with a plurality of bores positioned at a predetermined distance therebetween in the rotation direction, and a powder supply device for supplying powder in each bore of the turntable of the compression molding machine body. The powder is successively supplied in each bore of the turntable by the powder supply device while the turntable rotates, and the powder is compressed for molding in each bore by the time when the turntable completes its rotation of one circle, whereby tablets are successively produced.
A rotary powder compression molding machine in which an open feed shoe is provided in a powder supply device so as to be adjacent to the surface of a turntable of a compression molding machine body has been developed. According to this machine, powder is directly dropped onto the upper surface of the turntable on the upstream side of the open feed shoe in the rotation direction of the turntable.
In the rotary powder compression molding machine provided with this kind of powder supply device, powder is liable to form a bridge in a hopper of the powder supply device. Under this condition, the supply of the powder onto the turntable is prevented. When the powder is directly dropped onto the surface of the turntable, the holdup of the powder in the open feed shoe is substantially small. When the powder is prevented from being supplied from the hopper to the open feed shoe, the amount of the powder which will be loaded into the bore immediately runs short, resulting in the production of defective tablets whose weight does not reach a prescribed value. Moreover, the bridge formed by the powder in the hopper is merely maintained for a short period of time, and at the next moment after the supply of the powder from the hopper to the open feed shoe is terminated, the powder begins to be supplied again. Therefore, it is very difficult to detect the above abnormality under the supervision of operators and inspectors for the operation processes. Accordingly, when the powder is divided to be put into small packages, the shortage of the weight is found, causing great problems in production processes and quality control of the tablets.
As a method for preventing the production of these defective tablets whose weight does not reach a prescribed value, a method for detecting a shortage of the powder in the hopper by disposing a powder sensor in the hopper, a method for detecting a shortage of the powder which will be loaded into the bore by using the change of pressure when the powder is compressed for molding in the bore, and the like have been known. However, according to the former method, the defective tablets caused by the temporary interruption of the supply of the powder when the powder forms a bridge in the hopper cannot be prevented. According to the latter method, the apparatus becomes too large to be applied to the existing machine.
Japanese Utility Model Publication No. 57-50160 discloses a method for regulating the holdup of the powder in the open feed shoe, in which the level and the density of the powder held in the open feed shoe are detected to regulate the amount of the powder so that the holdup of the powder in the open feed shoe is maintained constant. In this method, the powder supply device is forcibly transferred from one end of the open feed shoe to the other end thereof, and the powder is supplied onto the turntable. Because of this, the holdup of the powder in the open feed shoe is greater than that when the powder is supplied onto the turntable from the hopper, and defective tablets caused by the bridge are not likely to be produced. However, in this method, the level and the density of the powder transferred by the open feed shoe are measured, so that it is not possible to prevent the tablets whose weight does not reach a prescribed value from being produced when the powder is not supplied onto the turntable from the open feed shoe.